Drop-in centers provide an invaluable safety net for homeless youth, addressing their basic needs for food, hygiene, and clothing. In addition, drop-in centers offer case management and other higher level services to address the interrelated health and social service needs of substance using homeless youth (e.g., substance abuse and HIV prevention programs). Research shows that homeless youth who access substance abuse, mental health, and case management services through a drop-in center are more likely to show reductions in substance use, improvements in mental health, and transitions to stable housing compared to youth who do not access these services. Yet nearly 40% of homeless youth do not utilize drop-in centers and, among those who do, an estimated 50% do not take advantage of higher level services such as case management and risk reduction programs. Virtually nothing is known about the barriers and facilitators of drop-in center utilization in this vulnerable population to better engage substance using homeless youth in needed services. The overall aim of the proposed project is to advance an understanding of key barriers and facilitators to the use of drop-in center services and develop a set of recommendations that can be used by service providers to encourage homeless youth to not only utilize drop-in centers, but to take advantage of higher level services that can reduce substance use and HIV risk behaviors. The specific aims of this project are: Aim 1) To develop a preliminary conceptual model of the facilitators and barriers of drop-in center utilization based on semi-structured interviews with homeless youth and service providers, and a systematic review of the existing literature on service utilization among homeless individuals; Aim 2) To conduct a survey with 200 randomly sampled homeless youth to examine whether hypothesized facilitators and barriers are associated with (a) any drop-in center attendance and (b) among users, frequency of attendance and types of services utilized (e.g., basic services vs. higher level services, includin substance abuse and HIV prevention programs); and Aim 3) To obtain feedback on our findings through semi-structured interviews with service providers to help ensure that the recommendations we develop are acceptable and feasible.